A systematic theory for the phenomena of determination and transdetermination among imaginal disks of Drosophila melanogaster is described. It has had considerable predictive success. It proposes that each disk's determined state is carried as the combination of states of several two-state "determination circuits", thus, wing equals (1111), leg equals (1010), etc. These combinations of states constitutes an epigenetic code requiring three control levels: positional signals to switch in the right state of each circuit in each region of the developing fly, the circuits themselves to maintain the determined state, and implementing processes which read combinations of circuit states and initiate disk specific development. The aims of the proposed experiments are both to test striking predictions of this combinatiorial model, and explore critical questions about the relation between homeotic mutants, transdetermination and developmental pathways.